The mechanism of burning rate enhancement of solid propellant compositions are generally classified as chemical or mechanical or a combination of each type.
Chemical enhancement of burning rate relates to either catalysis or chemical process interactions to yield increased burning rate, either or both of which may be influenced by or relate to surface phenomena, such as particle size, physical shapes, or mechanical interactions.
Mechanical enhancement of burning rate is, as the name implies a material that because of its shape, its distribution within a propellant matrix, and how it reacts under burning conditions can interact to affect or influence burning rate by heat transfer, by alteration of surface area or conditions, or by other physical interactions which influences the chemical and burning processes.
Various mechanical accelerators have been investigated. Some of these have been, (a) aluminum flakes, (b) aluminum staples, (c) aluminum whiskers, (d) graphite linters, (e) thermally-collapsible (shrinkable) tubings, sheets, rods, and hollow fibers, (f) microballoons, etc. Their use has been unsuccessful, when used in composite propellants, due to anisotropic burning characteristics of the propellant that these impart. The most recent material which has come to the fore as a mechanical accelerator is three-dimensional wire forms. The configuration of the wire forms is that of a paper staple in which one leg is at an angle of 90.degree. to the other leg.
The situation, insofar as composite-modified, double-base propellant is concerned, is different from that of composite propellants because of the methods of manufacture of the propellant. This process involves the use of casting powder in combination with casting solvent. When the casting powder is loaded into the motor, it is near randomly oriented, and when solvated by the casting solvent, this produces a propellant which undergoes isotropic burning.
In my copending application Ser. No. 06/382,005, filed May 5, 1982, I disclosed the blowing agent, 4,4'-oxybis (henzenesulfonyl hydrazide) (Celogen OT, manufactured by Uniroyal, Inc., 1230 Avenue of-the-Americas, NY, NY 0110), as an additive in an amount of about 1.0-3.0 weight percent to a composite propellant composition to achieve an increase in the burning rate as a result of undergoing decomposition when in contact with the flame front during propellant burning to rupture and thereby increase the surface area of the burning propellant surface.
The specified blowing agent is disclosed as equally effective in achieving a burning rate increase by a like mechanism when employed in a composite-modified, double-base propellant composition as an additive, or as a partial or complete replacement for aluminum whiskers employed in the casting powder portion with which a casting solvent portion is combined and cured to a propellant grain.
A further improvement for mechanical enhancement of the burning rate of solid propellants wherein Celogen OT in an amount from about 5 to about 8 weight percent of heat-expandable beads is additionally disclosed in my copending application Ser. No. 06/375,892, filed May 5, 1982. These heat expandable beads are incorporated into and physically dispersed within a composite or a composite-modified, double-base propellant composition in an amount from about 29 to about 4.0 weight percent of the propellant composition. These heat-expandable beads are comprised of thermoplastic styrene or its copolymers which are employed to encapsulate Celogen OT when styrene or its copolymers and Celogen OT are tumbled together. An organic peroxide is employed to catalyze the polymerization of the styrene and bead formation.
Although the heat-expandable beads of discrete particles of thermoplastic styrene containing Celogen OT as a blowing agent has been effective as a mechanical enhancer for the burning rate of the propellant composition, additional advantages could be achieved if the blowing agent were coated or encapsulated with a polymer that is chemically crosslinked in the propellant matrix rather than be just physically dispersed as discrete particles within the solid propellant matrix.
Therefore, an object of this invention is to provide gas-generated-expandable beads as burning rate accelerators which are physically dispersed within a solid propellant composition during the mixing procedure and subsequently chemically crosslinked during propellant composition curing procedure to form a solid propellant grain.
A further object of this invention is to provide gas-generated-expandable beads which are coated with the binder ingredient having the same chemical crosslinkers as those employed in the solid propellant composition so that after incorporating the coated gas-generated expandable beads chemical crosslinking of the beads ensures that the physically dispersed mechanical enhancers for burning rate are present as chemically hound entities in the cured solid propellant matrix which contribute to the increased density value and improved mechanical properties in addition to providing increased burning rate.
Still a further object of the invention is to provide a solid propellant composition with an improved burning rate while containing less carborane as a burning rate catalyst to thereby result in a less costly solid propellant composition as a result of containing a chemically crosslinked mechanical enhancer for burning rate as a direct replacement for a portion of the carborane burning rate catalyst.